This invention relates to printed circuits and their method of manufacture and, more specifically, to printed circuits comprising multiple layers of circuit elements each having rigid and flexible portions comprising; a sheet of flexible substrate material extending throughout the entirety of the rigid and flexible portions, paths of conductive material carried by at least one side of the sheet of flexible substrate material, and a flexible over-layer material extending over and attached to at least the entirety of all the flexible portions.
The printed circuits to be described hereinafter are generally very thin in cross-section being constructed of multiple layers of film materials (conductive, insulative, and adhesive). In the interest of ease of drawing and understanding only, it should be readily recognized and understood by those skilled in the art that the drawing figures which accompany the written descriptions are not to scale.
Printed circuits combining rigid and flexible portions are known in the art and are popular in many applications, such as automotive and aircraft. Such printed circuits allow modern printed circuit-mounted electronic components to be mounted and interconnected without the need for prior art wiring "harnesses" which were prone to damage, mis-wiring, and the like. As depicted in simplified form in FIG. 1 by way of example, such a combined printed circuit 10 may include several rigid portions 12 interconnected by flexible portions 14. Typically, the rigid portions 12 have components 16 and connectors 18 mounted thereon while the flexible portions 14 provide the interconnecting conductors 20 which replace the wires of the prior art wiring harnesses.
While the first printed circuits were typically a single layer of substrate having conductive portions formed on one or both sides, many printed circuits employed now comprise several conductive layers with the individual layers or elements being adhesively bonded together into a unitary structure. To provide inter-layer electrical connections, aligned holes through the layers (known as vias) are internally plated with a conductive material. Under ideal conditions, the foregoing structure and general method of manufacture presents no problems. Under typical manufacturing conditions, however, various problems present themselves. The result is a diminishing of the yield of the manufacturing process; that is, the various problems to be described shortly result in defects in the resultant printed circuits which make them unreliable and, therefore, unusable. As those skilled in the art are well aware, process yield is a most important factor in electronics manufacturing. Pricing and competitiveness (as well as product quality and reputation) depend on high yields of reliable components and products produced therefrom. Thus, it is of vital importance that the manufacturing processes used to make such multi-layer, combined rigid and flexible printed circuits result in high yields and constantly reliable parts.
Typical prior art approaches to the construction and manufacture of combined rigid and flexible printed circuits can be ascertained by reference to, for example, U.S. Pat. Nos. 4,687,695 and 4,715,928 (Hamby) and 4,800,461 (Dixon et al.). A typical prior art approach and its associated problems is depicted in simplified form in FIG. 2. As with the printed circuit 10 of FIG. 1, there is a multi-layer printed circuit 10' having rigid portions 12 and a flexible portion 14. At the core or center of the circuit 10' there is a substrate 22, usually epoxy-glass, having first conductors 24 formed on the outer surface thereof according to any of the many techniques known to those skilled in the art. The substrate 22 and first conductors 24 are protected on both sides by a flexible overcoating material 26 which is adhesively bonded thereto. The rigid portions 12 are created by attaching a rigid substrate 28 over the conductors and substrate 24, 22 employing a flexible adhesive material 30. The rigid substrate 28 also includes second conductors 32 formed thereon as necessary.
The rigid substrates 28 are typically attached using aligning holes through the various layers (not shown) with the second conductors 32 and the vias (shown in ghost as 34) then being formed in separate manufacturing steps. It is this approach which causes the manufacturing problems leading to reduced yield and reliability mentioned earlier herein. As a result of the thermal shock to which the circuits may be subjected in production and in testing processes, the considerable difference in the coefficients of thermal expansion of the adhesive material 30 relative to the various other materials tends to create voids in the adhesive material 30 and cracking in the conductive plating material in the vias 34. This can cause apparent defects which decrease the yield or, more seriously, latent defects which can cause the final product to fail or malfunction suddenly and unexpectedly at a later time.
Hamby, in both of the aforementioned patents, addresses the problems which result in failure due to thermal stress at plated through bores (vias) in rigid portions of printed circuits also involving flexible portions, by ensuring that, in the rigid portions of the printed circuit which encompass the vias, materials used are conventional rigid circuit board materials rather than the materials used in the flexible portions of the circuit. To this end, Hamby proposes that a sheet of conductive material, or a conductive pattern, be sandwiched between and bonded to sheets of flexible material in the flexible portions of the circuit and to rigid circuit board materials, such as epoxy-glass, in the rigid portions with the sheets of flexible and rigid materials abutting one another at the junctions between the flexible and rigid portions. The resulting structure involves substantial structural weakness at the junction of the flexible and rigid portions, due to the lack of continuity of the structural materials through this junction. At the same time the high rate of thermal expansion of materials such as epoxy-glass relative to the conductive (usually copper) layers results in a significant possibility of cracking and voids being formed particularly when the circuit board is subjected to significant thermal shock as well may occur during the formation of the vias interconnecting the various conductive layers of the circuit. Hamby cites difficulties in forming interconnects between layers of the printed circuit, particularly with the fabrication of the plated through holes, when polyimide materials are used in the region of the interconnects. Hamby does not appear to recognize the problems arising from the significant differences in thermal expansion rates of materials or the significant weakness attending the abutting relation of the materials at the junction between the flexible and rigid portions.
Dixon '461 builds on the teachings of Hamby, by using over-layer sheets slightly overlapping cut outs defining flexible portions of the circuit to reinforce junctions between the flexible and rigid portions. As a result of this, the flexible portions include flexible insulating materials which extend to but not a substantial distance into the rigid portions. Dixon goes to great lengths to avoid any significant presence of flexible materials, such as polyimide, in the rigid portions of the circuit and, in fact, emphasizes the use of epoxy-glass as the printed circuit substrate even in the flexible portions of the circuit referring to the use of a polyimide substrate only for use in flexible portion and only then when the flexible circuit portions need to provide maximum cable flexibility. The reason for this is to avoid the use of flexible materials, such as polyimide, in the rigid portions of the circuit. According to Dixon polyimide substrates, supporting the conductors in the flexible portion, must be terminated at the junction of the flexible and rigid portions with the consequent lack of structural integrity in this region. Dixon states that the use of polyimide in the rigid portions results in problems of thermal expansion and moisture retention and that such problems have been encountered in the past with polyimide materials where acrylic adhesives are used in the rigid portions. Dixon further states that his construction does not contain troublesome materials such as an acrylic adhesive and Kapton (a polyimide film made by E. I. duPont de Nemours and Co. Inc.), which, he states, have high expansion rates and moisture absorption problems. Dixon further states, in the introduction to his application, that the problems he encountered, in the then prior art rigid flex printed circuits incorporating flexible portions and plated through holes interconnecting conductive layers, were created by the thermal expansion of the then typically used insulated materials such as acrylic adhesives and Kapton. He states that failures occur when the board is subjected to elevated temperatures in thermal stress testing, hot oil solder reflow and the like, stating that, in the presence of such adhesives and polyimide materials, the copper in the plated through holes sometimes fractures and that repeated cycles of thermal stress tend to break many of the plated copper barrels formed in the vias of the rigid board section. Dixon also states that if less acrylic adhesive is used to limit expansion, internal stresses developed during lamination procedures cause unacceptable voids or delaminations in the final board. These are highly undesirable as they are, according to Dixon, not apparent until the final stages of construction with a result that costly scrapping of nearly completed boards is frequently required. Dixon attempts to resolve this problem by avoiding the use of acrylic adhesives and polyimide in the rigid portions of his boards, indicating that the use of such material is highly undesirable.
A further approach taken in the prior art which exacerbates the approach taken by Hamby and Dixon is the emphasis on maximizing peel strength between the layers of the circuit in each printed circuit element. This leads to the use of relatively sophisticated adhesives, such as acrylic adhesives, and results in a tendency to place the importance of peel strength above the importance of resistance to thermal shock and structural integrity at the junction between flexible and rigid portions of the circuit assembly.
It is an object of the present invention to provide a structure and a process which is predicated upon a reexamination of the basic parameters required in the effective design and production of printed circuit boards incorporating both flexible and rigid portions with a view to substantially increasing the yield of such structures, with the consequent economic improvement, while substantially eliminating the problems of voids and cracking resulting from thermal shock, maximizing the mechanical integrity of the junction between the flexible and rigid portions of the circuit, and, at the same time, providing an environment in the rigid portions of the circuit assembly for the effective plating-through of vias which will sustain thermal cycling without failure for a sufficient number of cycles to exceed the requirements of government standards in this respect.
It is a further object of the present invention to maximize economies in materials use by maximizing the uniformity of materials extending continuously throughout both the flexible and rigid portions of the circuit element. A secondary object with respect to this is the utilization of such continuous layers to provide effective protection against chemical damage, to existing parts of the circuit, during, for example, the production of the vias and subsequent conductor patterns on rigid parts of the substrate.